1. Field of the Invention
This invention relates generally to material removal, and more specifically to a method and apparatus for the electrochemical rifling of gun barrels and the like.
2. Description of the Prior Art
Electrochemical rifling is a well known process in which metal is removed from the inner cylindrical surface of a gun barrel or the like along circumferentially spaced spiral grooves by electro-chemical action. In the process, the metal barrel (anode) is connected to the positive post of a rectifier, and the metal rifling rod (cathode) mounted in the barrel is connected to the negative post. The rod has spaced radially extending insulating strips on its periphery engageable with the inner surface of the barrel. The working gap between barrel and rod is filled with an electrolyte fluid flowing under high velocity and pressure. The inner cylindrical surface of the barrel facing the uninsulated parts of the rod is electrochemically dissolved or removed by anodic action, that is, by current flow from the surface of the barrel through the electrolyte to the rod. The form and size of the uninsulated areas of the rod correspond directly to the form and size of the spiral rifling grooves in the barrel. The form and size of the insulated strips of the rod correspond directly to the form and size of the lands of the barrel extending between the rifling grooves. The amount of material removed depends primarily on current density and the process cycle time. The electrochemical action attacks only the anode or inner barrel surface, and the cathode rod surface shows no wear whatsoever.
A cathode rod used in the electrochemical rifling process described above comprises a cylindrical body preferably made from brass or copper. The body is provided on its periphery with a plurality of circumferentially spaced spiral grooves, and insulator strips of plastic or the like are mounted in the grooves. The outer diameter of the insulator strips is substantially equal to the inner diameter of the barrel for insulating the contiguous inner surface of the barrel from the electrolyte fluid during an electrochemical rifling operation. The electrolyte fluid flows through the spaces between the insulated strips and the uninsulated surfaces of the rod and barrel. The current flow from the cathode through the electrolyte to the anode in the uninsulated areas of the cathode rod causes the removal of metal material from the uninsulated areas of the barrel to form rifling grooves.
In the prior known apparatus and method for electrochemical rifling, it was necessary to provide seals between portions of the cathode rod and the barrel to be rifled to prevent the escape of electrolyte fluid which is subjected to a pressure of at least 300 lbs. per square inch. The barrels are held captive between two end caps, and one of the end caps has a small diameter bore for accommodating a cathode rod shaft by which the rod is moved relative to the barrel. With this type of apparatus, it is extremely difficult to provide small diameter seals within the bore surrounding the rod that are effective to resist the high pressure to which the electrolyte fluid is subjected. This problem is solved in the present invention by adding a long cylindrical tube in alignment with the barrel and encircling a portion of the rod. The area between the rod and tube support at one end of the tube can be more effectively sealed because of the larger sealing diameter. To assist in the sealing action between the relatively moveable cathode rod and tube, the surfaces of the rod and tube are ground to a very high finish having a clearance therebetween of around three-thousandths of an inch.
Still another problem encountered in the prior known method and apparatus for electrochemical rifling involves the generation of non-laminar electrolyte fluid flow between the rod cathode and inner surface of the barrel by virtue of the shape of the rod cathode head at its leading and trailing ends. The non-laminar flow causes electrochemical anodic action that is less effective resulting in poorer rifling. This problem was solved in this invention by providing the leading and trailing ends of the cathode rod with tapered or frusto-conical surfaces that reduce turbulence and provide laminar flow resulting in better rifling.
In the known cathode rod designs for electrochemical rifling, a problem is encountered when it is desired to rifle a barrel with a large number of grooves and lands such that the width of each land is very narrow. An example of this is rifling a 20 mm caliber barrel with 18 grooves and 18 lands. In such an application, it is virtually impossible to make precise narrow insulating strips and mount them in grooves on the cathode rod. This problem was solved in the prior art by mounting only a few insulated inserts of larger width on the cathode rod and rifling only a few of the grooves in one operation. A disadvantage of this solution is that it is then necessary to rotationally index the rod or barrel one or more times to complete the rifling operation. This results in greatly reduced productivity and less accurate rifling due to the difficulty of precisely rotationally indexing the rod or barrel. These problems are solved in this invention by providing a cathode rod that is divided into at least two contiguous aligned sections, each section adapted to machine half of the grooves to be rifled. Accordingly, the insulator strips can be made considerably wider eliminating the need to provide narrow precise plastic or insulating strips with the associated problem of mounting them. This invention greatly increases the productivity of the apparatus because all grooves are machined in one pass of the cathode rod through the barrel. Also, the accuracy is greatly improved because indexing and mistakes inherent with indexing are eliminated.